Tripping GFCI breaker, case study.

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mdshunk

mdshunk

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You guys tease me to no end about my preaching the use of the megger for troubleshooting, so I thought I'd share a quick story from a service call this morning.

On Monday morning, I did a panel change out. I took out an old FPE panel, which featured several GFCI breakers, and replaced it with a Square D panel. My usual custom is to install GFCI receptacles at the point of use instead of installing new GFCI breakers, but due to the fact that this customer had "custom" receptacle covers, I just put in Square D GFCI breakers.

I did warn the customer after the panel swap that due to the fact that they had FPE equipment before, and their noted ability to not trip when they should, they may experience tripping now due to genuine reasons that are just now being realized by the new equipment. Sure enough, they call late yesterday that one of the GFCI breakers is tripping and won't reset. These calls can be quite time consuming to sort out.

First thing, when I arrived this morning at the customer's home, the homeowner starts ranting about the breaker being "bad". Since my troubleshooting would start with removal of the panel cover anyway, I did replace the breaker while the customer was watching for the main purpose of proving to the customer that the breaker isn't "bad". On showing that the new breaker trips too (I knew it would), he retired to the house proper for the rest of my time there.

I took the hot and neutral off the breaker, and took the ground wire for that circuit off the ground bar. I checked hot to ground and neutral to ground with my ohm meter, and got completely open readings with the ohm meter. I put a megger on it at 500 volts, and got a reading of less than 5 megohms from hot to ground and neutral to ground. I forget the exact number, but it's not important. What is important is that it showed up on the megger.

This circuit was labeled "garage and outdoor receptacles", which I knew was correct because I labeled it myself 2 days prior. I quickly unplugged all items from these locations and did the megger check with the same results.

Experience tells me that outdoor receptacles are more likely to trip GFCI's, so that's where I started. Took the front outdoor receptacle out, and redid the megger check right from that location. Same bad results. Put front receptacle back together, and moved on to the back outdoor receptacle. Took the back receptacle out, and repeated the megger check at that location. SUCCESS! Good results now. Put the megger on the formerly removed receptacle by itself, and it tested bad. The receptacle looked visually fine, save for the normal wear an old outdoor receptacle shows, but it tested bad with the megger. I suspect it had drawn damp and maybe had an insect nest inside of it. Installed a new rear receptacle, and put the wires back on the GFCI breaker, and now it resets and stays reset.

Moral of the story, how would you have found that as quickly without a megger? Trial and error, I suppose? Even if you'd have replaced that rear outdoor receptacle and found that the GFCI breaker would stay set up, there would be no way for you to provide any assured "clearance testing" that the entire problem is solved without a follow-up megger check. My total time in that house was less than an hour. For something that can be as cheap as 100 bucks, it sped up my troubleshooting process and gives me surety that the job is completely and permanently done.
 
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dereckbc said:
Mark your an train driver right. What the heck are you doing with tools?:confused:
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Not sure I understand your first sentence. I had an HO train layout when I was a kid. That's the closest to engineer I ever got.
 
I was under the impression you were an engineer. But after checking your profile I see I am in error, your a fry cook at Mickey Dees.
 
Thanks for the informative post Marc, I'll now be looking for a megger. Your posts sure make a good case for owning one.:)
 
dereckbc said:
I was under the impression you were an engineer. But after checking your profile I see I am in error, your a fry cook at Mickey Dees.
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Nope, just a regular electrician. My avatar pic lends a little "artificial intelligence". :)
 
Uncle,Uncle

Uncle,Uncle

Alright you just convinced me to buy a megger. I have been reading the megger king posts for over a year now and you finally convinced me to buy one just now with this post. Congratulations!! it is probably the only tool I dont have now I am a tool guy anyway. Let me ask you though was the outside box wet inside or dry??
 
Marc,

Thanks for another imformative post.

I just started using a megger and am still not totally comfortable with it. I hadn't even thought about using it to check a recep. so thanks for that tip.

I do the same customer warning as you do on all panel swaps. I started this when I had a customer yelling and screaming at me on the phone that the breakers I installed were "to sensitive". :confused:

Jeff
 
Marc,
The megger is a great troubleshooting tool and I often use it. However, even 1 meg should not cause a GFCI to trip. That would only result in 0.12mA of current, far below the minimum trip level of 4 mA. Are meggers normally that far off in their readings?
Don
 
don_resqcapt19 said:
Marc,
The megger is a great troubleshooting tool and I often use it. However, even 1 meg should not cause a GFCI to trip. That would only result in 0.12mA of current, far below the minimum trip level of 4 mA. Are meggers normally that far off in their readings?
Don
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Don, I have a feeling that I dried some of the moisture with my test voltage. You can meg out wet stuff, hold the test button down (or set it for a one minute test) and watch the readings decrease as the test dries out the dampness. Brian John reports that he sometimes sees readings get "erratic" when wet cable is encountered as well.
 
pbeasley said:
OK, since everyone is jumping on the megger bandwagon, can you recommend a reasonably priced unit for occasional use for exactly the testing you described above?
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I guess the two budget models that I've been recommending are the Supco M-500 and the Extech 403360. Both are around 100-125 bucks. The "next step up" could be many different choices. These two are dirt-simple entry level ones.

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mdshunk said:
I guess the two budget models that I've been recommending are the Supco M-500 and the Extech 403360.* Both are around 100-125 bucks.* The "next step up" could be many different choices.* These two are dirt-simple entry level ones.**
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I bought the Extech 380260 a while back but I haven't used it yet. Although I have done a lot of hipot testing of medium voltage cables in my past I have never meggerred anything. I always skipped the megger test and went straight to hipot.What I don't understand is how to set up a 500 volt megger test on a circuit in a home where there are presumably numerous things plugged into receptacles and we don't necessarily know where all the receptacles are. Seems that would send 500 volts through whatever is plugged into the receptacles. What am I missing?
 
mkgrady said:
.What I don't understand is how to set up a 500 volt megger test on a circuit in a home where there are presumably numerous things plugged into receptacles and we don't necessarily know where all the receptacles are. Seems that would send 500 volts through whatever is plugged into the receptacles. What am I missing?
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In this case, I was checking hot to ground and neutral to ground. If I would have checked hot to neutral, I would have potentially blown up whatever happened to be plugged in.

The neutral to ground and hot to ground is how you also check the cord end of an old basement or garage refrigerator that intermittently trips a GFCI. I guarantee you that you'll find your leakage culprit if you megger check the cord of the fridge or freezer from neutral to ground or hot to ground.
 
Marc, just to be sure. You left the wires off at the panel and when you checked the front receptacle. Everything has to be unplug from the circuit you are checking right.
 
guschash said:
Marc, just to be sure. You left the wires off at the panel and when you checked the front receptacle.
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Yes.

guschash said:
Everything has to be unplug from the circuit you are checking right.
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I did unplug what I could find, but it is absolutely not necessary if you're just checking from neutral to ground or hot to ground. Matter of fact, it would be beneficial to leave things plugged in for an initial check, as I did, to rule in or out cord and plug connected equipment as the offender.

There are certainly instances where you want all the loads disconnected. These are the instances where you'd be Meggering from hot to neutral. You'd typically only megger hot to neutral when, for instance, you have a regular breaker tripping, and you suspect a cable fault someplace.
 
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you should rename this thread "megger 101"

In old homes where you are not sure that some DIY might have been DI themseves it is always wise to unplug everything.
 
Thanks Marc for the quick reply. I would like to start using my megger and I want to understand it before doing. So if say the CB is tripping, I would unhook the leads in the panel and hook them up to the megger. And set megger to 500v. If verything is good I should good no reading and if I get a read there is a problem with the insulation, correct.
 
guschash said:
Thanks Marc for the quick reply. I would like to start using my megger and I want to understand it before doing. So if say the CB is tripping, I would unhook the leads in the panel and hook them up to the megger. And set megger to 500v. If verything is good I should good no reading and if I get a read there is a problem with the insulation, correct.
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Not so fast. You really need to understand what leads you're hooking to the instrument and why. If you hook the hot and neutral up, and press test, and there are connected loads, you might have just done some damage. Check for connected loads first with an ohm meter or a megger check at low voltage (50 or 100 volts). If all is clear, then give it a rip with the regular test voltage. If you get bad readings (bad is subjective here) then you have a fault of some sort, and that's all you can say for now until you start troubleshooting. Could be a cable fault, device fault, equipment fault, etc. It is almost always safe to check from hot to ground and neutral to ground with connected loads. Sometimes, that's even desirable to determine if the connected loads have the ground leakage you might be hunting. Minding the test voltage, however, is fairly important because you can create leakage where none existed before. You wouldn't want to meg out an old fridge, for instance, at 2500 volts to check for leakage, because you'd surely create some.
 
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